Unity gain voltage buffers may be used to transfer voltage from a first circuit to a second circuit without the second circuit unacceptably loading the first circuit and interfering with the operation of the first circuit. In these circuits, the output voltage is an exact replica of the input voltage. A prior art unity gain voltage buffer circuit 10 is shown in FIG. 1. The input voltage VIN 14 and output voltage VOUT 24 are compared at the differential amplifier stage 40. The difference between these voltages is amplified and drives the output stage 42, which produces output voltage VOUT 24, which is a replica of VIN 14. Current for providing VOUT 24 is supplied to the output stage 42 by a secondary supply voltage 12.
The circuit 10 is shown in greater detail in FIG. 2. Transistors 16, 18, 28, 32, and 20 all form the differential amplifier stage comparing VIN 14 and VOUT 24 (a feedback voltage). The difference is amplified and presented at node 30, which drives the output stage 42 of the circuit 10. The output stage 42 consists of transistors 34 (the gate of which is coupled to node 30) and 36.
Five voltage sources are available to these types of circuits. There are the two major supply voltages common in all basic circuit: the main supply voltage VDD, which generally ranges from 2 to 4 V; and ground voltage, which is 0 V. A third voltage source is an auxiliary or secondary supply voltage VPP, which is produced by a positive charge pump that produces a voltage range of 4.5 V to 5.5 V (though other voltage ranges may be produced in other examples) with limited maximum output current. The limited output current is directly related to the amount of layout area on a chip which is allotted to the charge pump producing VPP. In addition, the output current is also limited due to the total active power consumption specification of the entire chip because the maximum output current is a percentage of the current the charge pump draws from VDD. The auxiliary supply voltage is also used in may other critical areas of the chip. The fourth voltage source is a reference voltage VREF, which allows for regulation of current flow in an analog circuit. The fifth and final voltage source is input voltage VIN, which in this example ranges from 1 V to 3 V. The input voltage cannot have a current load if it is to maintain its correct value.
The output voltage which replicates the input voltage ideally should be able to manage a large current load. This output voltage has a default state of 0 V. Returning to FIG. 2, since the value of VIN 14 can be higher than VDD, the secondary supply voltage VPP 12 is used to supply both the differential amplifier stage 40 and the output stage 24. However, this configuration results in a disadvantageous current load on VPP 12. Since there is a limited maximum output current for VPP 12, a larger output current requirement for VOUT 24 will drop VPP 12 in voltage and cause this circuit 10 and the other blocks using VPP to malfunction.
One potential solution to this problem is to increase the current output capability of VPP. This would result in a much larger charge pump which would take up more area on the chip. However, the active current specifications of the chip limit the size of the charge pump.
Therefore, it would be advantageous to provide a circuit that outputs a replica of its input voltage with increased current drive regardless of whether the input voltage is greater than the main supply voltage or if the main supply voltage or input voltage varies significantly.